This invention generally relates to electronic tuners of frequency synthesizing type, and more particularly to a type thereof wherein resistors and capacitors are used in a connecting part between the local oscillator circuit and the prescaler of the tuner such that the separation between VHF and UHF bands is improved.
In a television tuner of a frequency synthesizing type, it is an ordinary practice to apply the output frequency of a local oscillator to a frequency divider otherwise termed prescaler which divides the frequency into a value adapted to be processed in a digital circuit.
For instance, in a television tuner of frequency synthesizing type shown in FIG. 1, two local oscillators 1 and 2 are used for VHF and UHF bands, respectively, and the local oscillators are connected through a circuit 4 termed combiner which includes two constant k-type filters made of inductors and capacitors to the prescaler 3.
However, in a case the television tuner is used in a wide-band application for receiving CATV or the like besides the VHF and UHF bands, the high frequency range of the local oscillation circuit for the VHF band is closely adjacent to the lower frequency range thereof for the UHF band, and therefore the combiner 4 of the above described construction cannot be used.
More specifically, in an ordinary case where VHF and UHF bands of television broadcast are received, since the oscillating frequencies of the local oscillators are widely separated, the combiner 4 having a frequency characteristic as shown in FIG. 2 and operable as a frequency splitter can be utilized. However, in a case of a wide-band application for receiving, for instance, CATV, the frequency ranges of the tuner for these bands are closely adjacent or in an overlapping relation, so that the combiner or splitter 4 of the frequency characteristic shown in FIG. 2 cannot be used any more. In the latter case, a combiner or splitter 4 of a characteristic as shown in FIG. 3 is required to be used, which in turn causes a disadvantage of making it difficult to maintain independent operations of the local oscillators 1 and 2 in those frequency ranges adjacent to each other or overlapping with each other, but causing interruption or absorption phenomenon of the oscillators.
Furthermore, in a case of a wide-band tuner including three or more local oscillators, one for a lower VHF band, one for a higher VHF band, and others for the UHF band, the above described difficulties still exist, and therefore the circuitry of the electronic tuner is complicated, for instance, by the necessity of providing a band-pass characteristic for the LC splitter for the local oscillator provided for the higher frequency range of the VHF band.
For obviating these difficulties, there has been proposed a construction shown in FIG. 4 wherein local oscillators 1 and 2 are connected with the prescaler 3 through respective series connected circuits of a capacitor 5 and a resistor 6, and a capacitor 7 and a resistor 8. With this construction, not only the frequency characteristic shown in FIG. 3 can be obtained, but also a disadvantageous feature such as one of the local oscillators becomes the load of the other local oscillator can be eliminated by the existence of the resistors 6 and 8. In other words, adverse mutual actions between the VHF and UHF oscillators can be substantially eliminated.
In a case where the capacitors 5 and 7 in FIG. 4 are placed on the side of the local oscillators 1 and 2, and the resistors 6 and 8 are located on the other side adjacent to the prescaler 3, the above described connection may be schematically depicted as shown in FIG. 6 wherein only one half comprising the local oscillator 1, prescaler 3, capacitor 5 and the resistor 6 is indicated for the simplicity of the drawing.
As shown by dotted lines in FIG. 6, stray capacitances are formed between copper foils 9, each connected with the capacitor and the resistor, and the ground. Accordingly, as is apparent in FIG. 4, a series connected circuit of the capacitor 5 and one of the stray capacitances constitutes a load of the local oscillator 1, while another series connected circuit formed by the capacitor 7 and the other stray capacitance constitutes a load of the local oscillator 2. As a result, the capacitance in each tuning circuit for the local oscillator 1 or 2 increases, thus reducing the variable range of the oscillation frequency. In a worst case where the increase of the capacitance is excessive, the oscillation of the local oscillator tends to be interrupted in a range where the tuning voltage is low. Such a phenomenon is not advantageous for a wide band tuner for receiving CATV or the like, wherein a wide variable frequency range is required.